The present invention relates generally to semiconductor processing technology, and more particularly to a method for forming an integrated circuit (IC) having both high voltage and low voltage devices.
Many of today's IC's have some devices operating at a low voltage and other devices operating at a high voltage. For example, a liquid crystal display (LCD) driver is one of those IC's that have both low and high voltage devices. In order to operate in a high voltage condition, the metal-oxide-semiconductor (MOS) transistors of the LCD driver need to sustain high voltages without experiencing breakdown or parametric shifts. Although the breakdown voltage of the MOS transistor can be increased by using a thick gate oxide, hot carrier injection between its source and drain remains a concern. A variety of specialized transistor structures have been developed to minimize this hot carrier effect.
A double diffused drain (DDD) MOS transistor is one of the structures typically used for high voltage operations. The source/drain region of the DDDMOS transistor is constructed by two overlapping regions doped with impurities of different densities. Such DDDMOS transistor can provide a high breakdown voltage to better sustain an electrostatic discharge. The DDD structure can also provide a solution to the hot carrier effect when the transistor operates in a high voltage condition.
Conventionally, the processes of constructing the DDDMOS transistor operating at high voltages are incorporated in the typical processes of forming low voltage MOS transistors. This approach requires extra masks and ion implantation steps, thereby increasing the costs of manufacturing an IC with both the DDDMOS and low voltage MOS transistors.
It is therefore desirable to have methods for integrally constructing high and low voltage devices without the penalty of additional masks and processes that drive up the manufacturing costs.